1) Field of the Invention
The present invention relates to the generation of power from solar energy, and, in particular, relates to a solar power system having both trough and tower solar absorption devices.
2) Description of Related Art
Two conventional solar power generation systems are the trough- and tower-type solar-powered electrical generation plants. In a trough solar power plant 10, as illustrated in FIG. 1, a pump 12 circulates a heat transfer fluid such as hydrocarbon or synthetic oil through a fluid circuit 14 including a plurality of pipes 16 and trough receivers 18. Each trough receiver 18 includes an absorber tube for circulating the heat transfer fluid and an evacuated glass cylinder that surrounds, and thereby insulates, the tube. Parabolic sun-tracking mirrors 20, referred to as collectors, are configured to reflect sunlight toward the receivers 18 to heat the fluid therein. The heated fluid is then delivered to a steam generator 22, in which thermal energy is exchanged from the heat transfer fluid to water circulating in a separate fluid circuit 24. The heat transfer fluid is thereby cooled in the steam generator 22 and can then be re-circulated to the receivers 18 for reheating. The water heated in the steam generator 22 forms steam that is circulated to a turbine generator 26, i.e., a turbine 28 coupled to an electrical generator 30. The steam expands and rotates the turbine 28 and the generator 30 and thus produces electricity. The steam can be passed through a condenser 32 that, in conjunction with a cooling tower 34, condenses the steam to form hot water that is further heated in a preheater 33 and can be circulated back to the steam generator 22 by a pump 36 for re-use.
The pipes 16 and receivers 18 of the circuit 14 containing the heat transfer fluid lose thermal energy to the environment. The loss of energy generally increases as the temperature of the heat transfer fluid is increased beyond the ambient temperature. Thermal losses from the fluid circuit 14 can be reduced by insulating the pipes 16 and coating the receivers 18 with a selective coating, i.e., a material characterized by a low emissivity that maintains high thermal absorptivity. However, conventional selective coatings are destroyed by prolonged exposure to excessive temperatures. To avoid destruction of the selective coating, the heat transfer fluid in a trough solar power plant 10 is typically only heated to a maximum temperature of about 750° F. or less, thereby limiting the temperature of the steam generated to about 700° F. The maximum operation temperature is also limited according to the oil or other heat transfer fluid that is used, as conventional oil fluids break down or undergo phase changes at temperature above about 750° F. Turbine generators operate at higher efficiencies at higher operating temperatures, and typically operate at an efficiency of about 32% for the typical operating temperature of a trough solar power plant.
Higher operating temperatures can be achieved in a tower-type solar power plant 40, as illustrated in FIG. 2, in which the heat transfer fluid is circulated to a tower 42 that includes an absorption device, also referred to as a receiver 44. The receiver 44 of the tower 42 includes serpentine tubes that receive the heat transfer fluid. A plurality of heliostats 46 are configured to reflect sunlight toward the receiver 44, thereby heating the fluid within. Each heliostat 46 is a sun-tracking mirror configured to change position according to the direction of the sun 106 so that the heliostats 46 reflect solar energy onto the receiver 44 despite the continuous movement of the sun 106. The heat transfer fluid is delivered from the tower 42 to a steam generator 48, where thermal energy is exchanged to water, forming steam and generating electricity as described above using a turbine 50 and electrical generator 52. The steam is then condensed in a condenser 54 with the use of a cooling tower 56, and the water is preheated in a preheater 55 and re-circulated to the steam generator 48. Pumps 58 can be used to circulate the heat transfer fluid and the water in the respective fluid circuits, and tanks 60, 62 can be used to store the heat transfer fluid before and after heating by the receiver 44, respectively.
The surface area of the receiver 44 of the tower solar power plant 40 is typically significantly less than the surface area of the receivers 18 of the trough solar power plant 10. Thus, the receiver 44 of the tower-type solar power plant 40 generally suffers less heat loss than the receivers 18 of the trough solar power plant 10, even without the use of low emissivity coatings, and despite operation at higher temperatures to about 1050° F. Further, since the tower 42 can typically heat the heat transfer fluid to higher temperatures, steam can be heated to about 1000° F. The heat transfer fluid can be a molten salt, which is chemically stable to temperatures of 1100° F. and has a high boiling point. However, such salts generally have freezing temperatures greater than those of oils typically used in trough solar power plants. Therefore, if sufficient solar power is not available for heating the molten salt, the fluid is typically electrically heated or drained from the receiver 44 to prevent the salt from solidifying therein.
The higher steam temperature associated with the operation of the tower solar power plant 40 allows the turbine 50 and electrical generator 52 to operate at a higher efficiency than that achieved with the lower temperatures associated with the trough solar power plant 10. Thus, the tower solar power plant 40 can achieve a higher overall efficiency, but the trough solar power plant 10 costs less to operate at low operating temperatures, i.e., up to about 750° F.
Thus, there exists a need for an improved solar-powered power generation system. Preferably, the system should be capable of heating a transfer fluid to high operating temperatures typically associated with tower solar power plants. The system should also provide inexpensive heating of the transfer fluid, as is associated with typical trough solar power plants.